In a typical telemanipulation system that does not support force reflection or compliance control, a stiff remote manipulator moves strictly according to a human operator's position command, and small errors between the actual and the commanded position of the manipulator can give rise to undesired large contact forces and torques. It is thus hard to expect safe and reliable telemanipulation with this system.
Two major techniques that alleviate this excessive contact force problem are force reflection and shared compliance control. [A. K. Bejczy, Z. Szakaly and W. S. Kim, "A Laboratory Breadboard System for Dual-Arm Teleoperation," Third Annual Workshop on Space Operations Automation and Robotics (SOAR '89), pp. 649-660, NASA Johnson Space Center, Houston, Texas, July, 1989; and W. S. Kim, B. Hannaford and A. K. Bejczy, "Force-Reflection and Shared Compliant Control in Operating Telemanipulators with Time Delay," IEEE Trans. on Robotics and Automation, Vol. 8, No. 2, pp. 176-185, Apr. 1992]
In force reflecting teleoperation, the operator can feel contact forces and torques through a force reflecting hand controller and thus adjust the hand-controller position naturally to reduce undesired contact force components. Experimental studies indicate a significant enhancement in the human operator's task performance with force reflection. In shared compliance control, the operator's commanded position is altered by a compliant control force feedback in the robot side. This local autonomous force feedback in the robot side adds active compliance and damping to the stiff robot hand, making the robot more compliant to the environment and softening mechanical contacts/collisions between the manipulator and objects. Recent experiments demonstrated that shared compliance control is essential in time-delayed telemanipulation [Kim, et al., supra].
Recently, orbital replacement unit (ORU) change-out experiments were performed with the JPL/NASA telerobot testbed system [W. S. Kim, P. G. Backes, S. Hayati and E. Bokor, "Orbital Replacement Unit Change-out Experiments with a Telerobot Testbed System," IEEE Int. Conf. on Robotics and Automation, pp. 2026-2031, Sacramento, Calif., April, 1991], and the experimental results showed that without shared compliant control (SCC) or force reflection (FR), the operator could not complete the task, while with SCC or FR the operator could perform the task successfully with reduced contact forces both in magnitude and duration. The results also indicated that the task performance with SCC was superior to that with FR in terms of task completion time, cumulative contact force, and total contact duration. The relatively poor performance with FR was mainly due to a poor force reflection gain.
A major advantage of FR is that the operator actually feels the contact forces/torques sensed by the telerobot hand. However, the maximum FR gain attainable in this telerobot testbed system without causing instability has been approximately 1/10. With this low gain, the operator could feel only 1 lb when the manipulator hand senses a 10 lb contact force. The problem of poor force reflection is not specific to this testbed system, but rather inherent to the conventional FR control scheme being used for dissimilar master-slave systems where the slave system usually has much higher stiffness than the effective stiffness of the human hand holding the force reflecting hand controller.
In a typical force-reflecting telemanipulation system consisting of dissimilar master-slave arms, the position of a slave arm (remote manipulator) is controlled by the human operator command, HO, through a master arm force-reflecting hand controller 10 as shown in FIG. 1, while the contact forces/torques sensed by the force/torque sensor of the robot servo system 11 at the base of the robot hand are reflected back to a human operator through the master arm of the hand controller 10. This forms a closed-loop system and raises a stability issue. Existing force-reflecting systems supporting dissimilar master-slave arms show that the force-reflection gain from the robot hand to the force reflecting hand controller is limited to approximately 1/10. This poor force reflection problem will now be discussed.
As a first-cut rough approximation, a linear decoupled system model in Cartesian axes is assumed. In FIG. 1, the open-loop transfer function Q(s) is given by EQU Q(s)=G.sub.ps G.sub.fr K.sub.me H(s)R(s), (1)
where G.sub.ps is the position command scale factor, G.sub.fr is the force reflection gain, and K.sub.me is the effective stiffness which is a parallel combination of the manipulator stiffness and the environment stiffness. R(s) is the robot servo system transfer function in Cartesian space [W. S. Kim and A. K. Bejczy, "A Stability Analysis of Shared Compliance Control," Japan-U.S.A. Symp. on Flexible Automation, pp. 567-572, Kyoto, Japan, July 1990] and is given by a linear sum of the six second-order joint servo transfer functions with the DC gain of R(O)=1. R(s) could be second-order, fourth-order, or higher depending upon the Cartesian axis and the arm configuration. An example of a Cartesian space frequency response of the PUMA arm used in the Advanced Teleoperation System is shown in FIG. 2. In this example, the double-pole corner frequencies are at about 3 and 6 Hz, behaving as a fourth-order system. H(s) is the transfer function of the operator's hand holding the 6-degree-of-freedom force-reflecting hand controller [Bejczy, et al., supra]. The transfer function can be obtained by measuring the magnitude ratio of the hand controller deflection to the applied force input for different frequencies. Measurements indicate that the compliance value C.sub.h (=H(O)) varies from about 1.0-2.0 in/lb (0.5-1.0 lb/in stiffness) with a loose grasp to about 0.1-0.2 in/lb (5-10 lb/in stiffness) for a firm grasp. The bandwidth of H(s) is about 1 Hz for a loose grasp and 3 Hz for a firm grasp. Typical frequency responses of the operator's hand holding the force reflecting hand controller for firm grasp (circle) and for loose grasp (triangle) are shown in FIG. 3. In order to have a stable teleoperation system with a constant force reflection gain G.sub.fr, the open-loop DC gain Q(O) should not be much greater than 1, since a higher-loop gain causes instability due to the higher-order dynamics of H(s)R(s). Namely, ##EQU1## In a typical system, the combined stiffness of the manipulator and environment is measured K.sub.me =25 lb/in, and it is assumed that the operator's hand can maintain at least a 2.5 lb/in stiffness (C=0.4 in/lb) during teleoperation. In this typical situation, the manipulator/environment stiffness is much higher than the operator's-hand/hand-controller stiffness (K.sub.me C.sub.h 10), and from Equation (2) the maximum force reflection gain G.sub.fr is limited to only 1/10 for the unity position scaling factor (G.sub.ps =1). The foregoing analysis clearly indicates that the poor force reflection is not due to a poor implementation of the specific systems, but rather inherent to the existing conventional force-reflection system with dissimilar master/slave arms, when the bandwidth of the robot servo system dynamics R(s) is not substantially higher than 3 Hz which is the approximate bandwidth of the operator's hand dynamics with the hand controller H(s). A good direction to increase the force-reflection gain is to make the robot more compliant by employing compliant control.
Shared compliance control has been implemented in the prior art [Kim, Hannaford and Bejczy, supra] by low-pass filtering (LPF 12) the contact force (outputs of the force/torque sensor mounted on the base of the robot) and using these signals to alter the human operator's position/orientation command (HO/HC) received by the robot servo system 11 as shown in FIG. 4 using a mixer an adder/subtractor, hereinafter referred to as 13 which adds a negative force feedback signal to a positive HC position signal or subtracts the force feedback signal from the HC position signal if both are prepresented as positive or negative signals, as may sometimes be the case in digital signal systems as opposed to analog signal systems. This low-pass-filtered force/torque feed an effect of giving the robot hand behavior similar to a damped spring (in each of the task space dimensions) in series with the stiff, positioning-controlled, robot manipulator. An approximate mechanical equivalent of the above implementation consists of a spring connected in parallel with a damper. It can be shown that the compliance control force feedback gain G.sub.cc is approximately the new compliance value of the compliant robot control system of FIG. 4.